S-3-Aminomethyl-5-methylhexanoic acid, i.e. the pregabalin (PGB, the trade name Lyrica), the chemical name of which is (3S)-3-aminomethyl-5-methylhexanoic acid, the molecular formula of which is C8H17NO2, the molecular weight of which is 159.23, is a white crystalline powder, and has a melting point of 184° C.-186′C. The pregabalin is a novel-type γ-aminobutyric acid (GABA) receptor agonist, can block voltage-dependent calcium channels and reduce the release of neurotransmitters, and is used in clinical for the treatment of the peripheral neuropathic pain, the pain caused by the diabetic peripheral neuropathy (DPN), the postherpetic neuralgia (PHN), and the ancillary treatment of part of the epileptic seizures.
The structural formula of the pregabalin is shown as follows:

Pregabalin is an analogue of the neurotransmitter GABA, has the fat-solubility, and can pass through the blood-brain barrier. The mechanism is to reduce the calcium influx by inhibiting a subunit, the a2-σ protein, of the voltage-dependent calcium channel of the central nervous system, so as to reduce the release of the excitatory neurotransmitter substances such as glutamate, norepinephrine, P substance and so on. For the racemic compounds of 3-aminomethyl-5-methylhexanoic acid, in the aspect of binding to these sites, the activity of the isomer (R) is only 1/10 of that of the isomer (S).
The U.S. Pat. No. 5,637,767 disclosed a method of preparing S-3-Aminomethyl-5-methyl hexanoic acid, and the specific process is described as follows: the racemic 3-aminomethyl-5-methylhexanoic acid and the S-mandelic acid are dissolved in a 97% (v/v) isopropyl alcohol aqueous solution and maintained at that temperature for a period of time, cooled and seeded to crystallize, then filtered to obtain the first resolution wet product of S-3-Aminomethyl-5-methylhexanoic acid-5-mandelic acid salt, after pumped dry, the wet product is added again into a 97% (v/v) isopropyl alcohol aqueous solution, to make the second resolution by adding a certain amount of S-mandelic acid, to obtain the resolution product of S-3-Aminomethyl-5-methylhexanoic acid-S-mandelic acid salt, after dried, the resolution product is heated to reflux in the 95% (v/v) THF aqueous solution and maintained at that temperature for a period of time, then cooled and filtered to obtain the crude product of S-3-Aminomethyl-5-methylhexanoic acid, after pumped dry, the crude product is refined directly in a 75% (v/v) isopropyl alcohol aqueous solution to obtain the fine product of S-3-Aminomethyl-5-methylhexanoic acid, i.e. the fine product of the pregabalin, and the total yield is about 25%.
The above-mentioned resolution process adopts the isopropyl alcohol as the solvent, and the S-mandelic acid as the resolution agent, for the S-mandelic acid is expensive, the cost is high, it further adopts the THF aqueous solution to hydrolyze, thus it is difficult to recover, and the pollution is serious, moreover the overall yield is low, and the purity is not high.